1. Field of the Invention
The invention relates to the refining of tall oil and more particularly relates to the separation of neutrals from tall oil soaps.
2. Brief Description of the Prior Art
Tall oil is a well-known by-product of the sulfate or kraft process for digesting wood, obtained by the acid conversion of black liquor soap skimmings. Crude tall oil is made up of varying proportions of fatty acids, resin acids and so-called "neutrals". Crude tall oil obtained in the United States typically comprises from 18 to 53% by weight of fatty acids, 35 to 65% by weight of resin acids and 8 to 24% by weight of neutral compounds. The presence of "neutrals" in tall oils is undersirable, e.g., since the presence of neutrals can adversely affect processing of tall oil product. These compounds comprise a wide variety of alcohols, aldehydes, ketones, hydrocarbons, sterols and like compounds which may interfere with certain uses of applications of the fatty acids and/or resin acids which comprise tall oil. However, some neutral species are only separated from tall oil by techniques such as by molecular or fractional distillations. Moreover, some neutral species cannot be separated efficiently from fatty or resin acids by distillation, because of similar volatility characteristics of some neutrals compared with fatty acids and other neutrals compared with resin acids. Also, existing distillation techniques are relatively severe, not only causing tall oil degradation, but also resulting in acid loss due to neutrals reacting with acids, e.g., esterification (alcohol-type neutrals).
By the method of the present invention, the neutrals may be separated from the tall oil soaps, i.e., the salts of tall oil acids which are the black liquor soap skimmings, to reduce their content therein. The soaps processed by the method of the invention, upon acid treatment, yield tall oil of a reduced neutrals content. Neutrals have been separated from tall oil soaps by extraction in water-immiscible hydrocarbon solvents; see for example U.S. Pat. No. 3,965,085. However, such processes require relatively difficult separatory steps to recover the solvent and separate the neutrals from the extracting solvent.
The employment of gases as liquids in their near critical state and as fluids in the supercritical state to serve as an extracting solvent has been previously described; see for example Francis, Physical Chem. 58,1099 (1954) and Ind. Eng. Chem. 47, 230 (1955). Near critical and supercritical fluids, including supercritical hydrocarbon gases, have been suggested as solvents for a wide range of materials; see for example U.S. Pat. No. 3,969,196 and British Pat. No. 2,032,789. Despite the fact that the solvation properties of gases in their near critical and supercritical states and especially of supercritical hydrocarbon gases, has been known, the application of this knowledge has not been applied to tall oil soap refinement.
Advantageously the process of the invention facilitates the separation of extractive solvent from the tall oil soap and simplifies the recovery of neutrals from the extracting solvent. The process of the invention also has advantages in the more efficient utilization of energy.